The use of single speed motors to drive elevator cars has been known in the art for a number of years.
In a typical usage, the elevator car was accelerated to the rated speed of the car. When the elevator car approached the desired floor, a stop was electromechanically activated at a predetermined distance from the floor.
In using a mechanical braking means to stop the elevator car, the stop position was dependent on load and the mechanical brake could not compensate for the variable loads and speeds encountered.
As a result, leveling of the car was often inaccurate and releveling was frequently required.
Because of the clearly dangerous conditions created by the prior art mechanical braking systems, the National Elevator Safety Code will soon require a more accurate leveling standard for single speed elevator cars.
Accordingly, it is an object of the present invention to achieve the accurate and automatic leveling of single speed elevator cars which will meet the standards of the National Elevator Safety Code.
It is a further object of the invention to utilize a solid state braking circuit which has been advantageously designed to decrease elevator car braking time as compared to prior art systems.
It is also an object of the invention to reduce the risk of injury to persons or goods which has arisen from the inaccurate leveling systems heretofore used in the art.
It is a still further object of the invention to accomplish the above goals while using a braking system which is economical to manufacture and which may be easily installed into existing elevator car systems.
It is also an object of the invention to control elevator motor speed by utilizing the external neutral connections of an AC motor for ease of installation and maintenance.
These and other objects and advantages of the invention will be clear to those of skill in the elevator control arts from the description which follows.